The research aims of this study are to relate the new areas of cytogenetics and human behavior genetics to problems in normal human development and psychopathology. We are investigating the nature of the relationship which might exist between the sex chromosome aneuploides in males (i.e. XXY, XYY) and subsequent personality development. Infants with these chromosomal lesions--and infants serving as controls--were located through a chromosomal screening of male neonates. These infants and children are being followed in a long-term developmental study with the aim of identifying reactive styles in the infants that might be related to the chromosomal variations. Considerable controversy has surrounded the issue of the possible relationship which might exist between certain sex chromosome variations and personality development. The controversy has resulted from the fact that much of the evidence suggesting such a relationship has been obtained by surveying groups of individuals who demonstrated in particular personality deviation and finding an increased incidence of a specific chromosomal aneuploidy among them. The natural history of unselected individuals who carry such variations remains largely unknown. Should long-term longitudinal studies of affected individuals reveal any developmental risk associated with the variation, this knowledge can be directed to helping these individuals overcome developmental lags and realize their full developmental potentials.